The accessory molecule Lgp55 plays a role early in murine fetal thymocyte differentiation.

A rat IgM monoclonal antibody, PA3-795, inhibits the antigen-specific responses of mouse T-cell hybridomas. It recognizes a heavily glycosylated cell-surface protein, designated Lgp55, that is detectable after activation on mature T cells. During fetal life, Lgp55 is found at high levels on newly immigrant thymic T-cell precursors prior to surface expression of other T-lineage molecules. High levels of expression are also found on thymocytes in the outer cortex of adult mice. Thymocytes at later stages of differentiation bear decreasing amounts of surface Lgp55, and none is detectable on "single-positive" thymocytes in the thymic medulla or on resting mature T cells from the periphery. Addition of monoclonal anti-Lgp55 to fetal thymus organ culture decreases the output of "mature" CD4 single-positive thymocytes when it is begun before fetal day 13.5. These findings suggest that Lgp55 contributes to cell-cell interactions that regulate very early steps in T-cell development in the mouse.